Steels are widely used in various field of industrial such as automotive, chemical and structural industries. Nevertheless the main obstacle of steel application in industries is the corrosion problems that diminish some desired properties. Recently there are developments of process to lessen the corrosion, additionally to improve aesthetic property for instance colors and glossiness to serve market needs except from ordinary colors. The zinc-coated steels cannot provide sufficient corrosion resistivity and aesthetic property. Thereby adding the second process of second coating on zinc-coated steel was introduced which is called Chromate conversion coating. In this work, for the first time, a systematic study is carried out to correlate the electroplating parameters, mainly the current density, on the formation of the trivalent chromate conversion coating, and hence the color appearance of the top-coats. This work focused on the study of current density at 0.5, 1.0, 2.0 and 4.0 Ampere per square decimeter. Focusing on the black conversion coating, since the black coating is one of the most widely used color in the market. The study had been processed on the basis of metallurgy, electrochemistry and optical engineering to find the correlations that current densities have towards structural and optical property of the films. Also the first time Hull cell setup had been introduced to fabricate the samples in order to create more ease of distinguishing the color and have all controlled conditions in one setup in one bath. The color and optical properties are analyzed using a colorimeter and an optical CCD spectrometer. Also comparison to visual inspection had been implemented to see the relation and to obtain virtual optical data The results notably show that, current density affects the optical properties in the visual spectrums. Not only the processing parameters from conversion coating that can affect appearance of it, but the processing parameters such as current density from zinc process also have influence towards conversion coating. It had been discovered that the current densities from zinc plating layer have the influence towards shade of black color on CCC layer changing, the high current density area exhibits darker blackness comparing to low current density area. Therefore controlling only process of conversion coating might not be sufficient. Controlling the process from first layer of zinc is also important in appearance wise. The basic microstructural and chemical characterization techniques, namely FE-SEM, OM, and XRF, are used to shed some light on the possible underlying mechanism that controls the color appearance. For further study, suggestions of investigation means had been advised to get more tangible and clearer microstructural data. The understanding developed in this study will impact the design and fabrication of the electroขgalvanizing products of desired color and esteemed functional performance.

Thesis (M.Eng.)--Chulalongkorn University, 2014